Turbofan engines are a dominant contributor to overall aircraft noise. Reduction of the turbofan engine noise is therefore critical for aircraft compliance with current and future noise regulations. Fan noise comprises a large component of the overall turbofan engine noise.
The continuing push towards lower thrust-specific fuel consumption implies increased by-pass ratios (BPR>6) with greater fan diameters, fan chord lengths, and consequently, lower fan rotational speeds. Fan blade design and contouring, through the use of lean and sweep, have been employed to modify the acoustic characteristics of the engine. These trends lead to a further shift of the fan noise spectrum to lower frequencies, accompanied by increased broadband noise as a significant contributor to the aircrafts community noise impact. Airframe drag reduction considerations imply engine nacelles that are shorter relative to their diameter, thus limiting lined-duct-treatment-to-diameter ratio. This trend, along with limitations on nacelle wall thickness, inhibits liner effectiveness since a deeper liner would typically be required to mitigate lower frequency noise.
Fan noise has traditionally been alleviated by a combination of passive liner treatments and nacelle modifications. Known passive liners may include a honeycomb core bonded between a porous face sheet and an impervious backing plate. This configuration produces an array of independent, one-dimensional, tuned waveguides that behave as local-reacting absorbers. The acoustic absorption spectra of such structures are characterized by a single peak at the system resonance frequency and its odd harmonics with significantly reduced absorption at other frequencies.
Single-layer perforate-over-honeycomb liners may be used for absorption of individual fan tone frequencies and their harmonics. Extension to include broadband sound absorption is generally achieved via multi-layer acoustic liners (generally 2, but sometimes 3 layers) and/or variable depth. These designs are driven by the fidelity of the prediction tools, as well as geometric constraints, including 1-D impedance prediction tools based on a transmission line approach and 2-D drawing software (e.g. Corel Draw®, MS Paint®, SmartDraw®, etc).
Typical approaches to liner design have focused on narrow-band attenuation spectra and are generally not broadband in character. Liners were developed to have constant depth which simplified the analysis. Creating a broadband liner design consisted of developing a liner geometry based on various acoustic rules and generally accepted practices then analyzing them with traditional command line style computer codes. The lack of integration between liner geometry design and analysis greatly increased the time to complete an iteration loop making such designs tedious.